The present invention describes improvements for many applications of previous inventions as described in U.S. Pat. No. 6,048,374 (374) and U.S. Pat. No. 6,830,597 B1 (597). The improvements presented herein are in part the result of many tests with process development unit (PDUs) of devices modified in various degrees from those described in 597 and 374. In large part the improvements reflect the result of system analysis studies particularly those using the PI's analytical semi-empirical model (ASEM) of pyrolysis of carbohydrate materials.
Tests carried out during the development of green pyrolyzer gasifier (GPG) systems have used electric ovens, charcoal ovens, oil ovens and gas ovens to provide the heat of pyrolysis. Most of the earlier tests focused on generating a clean gas to be used with small scale gas to electricity generators. However, after extensive experimentation with such systems it became clear that when using a pyrolysis gas fired motor generator to produce electricity, the output/input (O/I) is not favorable at this time. This is largely due to the inefficiency or high cost of currently available small scale gas to electricity converters. Mass and energy balance calculations, however, indicate that by using the pyro-volatiles, or pyro-chars to provide the heat of pyrolysis, a favorable heat power O/I can be obtained. Experimental tests with char heating and gas heating for feedstock pyrolysis have indicated that heating with the volatiles is simpler than heating with the char, and furthermore the collected char is a valuable product.
The system analysis type studies of ultimate and proximate analyses of materials along nature's coalification path [6, 8-13] have pointed to the importance of the oxygen weight percentage [O] and how the hydrogen [H] and carbon [C] are correlated with [O] among natural fuels. FIG. 1a illustrates the results of a large compilation of ultimate analysis [H] vs [O] and [C] vs [O] data obtained from many sources in the coal and biomass literature. Here the data has been corrected to dry, ash, sulfur and nitrogen free (DASNF) materials and ignores trace (ppm) elements. The formula [H]=6{1-exp-[O]/2]} provides an approximate smooth representation of the overall trend of [H] with [O] for these substances. For DASNF material [C]=100−[H]−[O]. The curve through the [C] vs [O] data points assumes this smooth [H] vs [O] relationship.
FIG. 1b shows the systematic of total volatile (VT) vs [O]. for DASNF materials. It is reasonably well represented by VT=62{([H]/6)*([O]/25)1/2}. For most plant matter [O] is around 45% and experiment or a simple calculation shows that the total volatiles released in high temperature pyrolysis is in the 80% range. Thus pyrolysis of plant matter is essentially a direct form of gasification. The fixed carbon, FC (FC=100−VT), is thus typically 20%. In contrast for bituminous coal [O]˜10% and our formulas give VT˜40% and FC˜60%. Table 1 lists some thermal properties of fuels along nature's coalification path.
Table 1 Properties of Fuels along natures coalification path. [C], [H], [O], VT and FC are in weight percentage. HHV is in MJ/kg. RelchR denotes relative char reactivity.
UltimateProximateAnalysisAnalysisOther propertiesName[C][H][O]HHVVTFCDensE/volRelchRH, OHORankAnthracite9433367931.6581.5v. low3-OBituminous855103533671.4495low10-OSub Bitum 75 5203051491.2 3616med20-OLignite70 52527584212750interm25-OPeat606342369310.818150high34-OWood497441881190.611500v. high44-OCellulose 446501088120.491600v v. high50-O
Higher heating values (HHV) are usually reported along with proximate analysis. FIG. 1c displays HHV data for the compilation of materials after correction to DASNF cases. Most points within this noisy data can be fit within a few percent byHHV=34.9−0.453[O]+0.829[H] in MJ/kg. or HHV=15.00−0.194[O]+0.356[H] in Btu/lb.This form of DuLong's formula is simplified from that used by Channiwala and Parikh [21].
The rule HHV=15−[O]/5+[H]/3 in Btu/lb should be good enough for ball park purposes. The smooth curve in FIG. 1c shows the trend of the HHV vs [O] curve when the smooth [H] vs [O] relationship is used. In applying these HHV formulas note that most plant materials have [H] near 6% whereas [O] is near 45%. Thus the negative [O] term generally has a greater influence than the positive [H] term.
FIG. 1b and the formula VT=62{([H]/6)*([O]/25)1/2} indicate that high [O] materials give high percentages of volatiles. However, this DuLong formula assigns low heating values to high [O]'s [12,13]. Blending feedstock to achieve favorable properties for pyrolysis could have a number of advantages [11-14]. Air blown partial combustion is a long established and still prevalent approach to biomass gasification. Unfortunately the HHV of its gaseous product is not only energetically reduced by the air's 20% oxygen but even further reduced by dilution with air's 80% non-energetic nitrogen so that the product is a low HHV producer gas. When pyrolysis is taken as the route for conversion of solid biomass the CO2 and H2O pyrolysis volatiles without energetic value are mainly released at lower temperatures. In the improvements described herein, these non-energetic volatiles can serve as assets.
The present invention is in large part based upon the teachings of an analytical semi-empirical model (ASEM) [13-20] that systematizes pyrolysis yield data extracted from the technical literature or measured by the PI's group. FIG. 2 illustrates examples of ASEM results for six representative solid materials along nature's coalification path [13]. The numbers on top of each box are the weight percentages (wt %) of carbon, hydrogen and oxygen, after correcting to dry, ash, sulfur, and nitrogen free (DASNF) conditions. One should note the scale changes and the fact that as the oxygen wt % goes up the yields of CO2, H2O, CO and Tars go up sharply. Here HC mostly stands for the sum of C2-C4 gaseous members of the paraffin, olefin, acetylene, diene, aldehyde and ether families. The Tars stand for the C5 and higher liquid and solid members of hydrocarbon families plus hundreds of oxygenated compounds (carbohydrates) that condense at standard temperatures. Providing approximate yields of these many products in analytic forms useful for engineering applications has been the goal of ASEM studies.
FIG. 3 illustrates typical char, tar and total gas pyrolysis products versus temperature curves for woody materials corrected to DASNF cases. The char yield is then primarily the residual carbonized feedstock after the pyrolysis volatiles are driven off. The volatiles consist of the sum of non-condensable volatiles (gases) and volatiles that condense at standard temperature (sometimes designated as tars). For the temperature used with GPGs, the condensable volatiles are mainly liquid carbohydrates or hydrocarbons at standard temperature, although some are solids like waxes and tar-like materials.
The need for renewable sources of fuels for the transportation sector and the need to mitigate climate change have been strong motivations for the development of GPG technologies. From the GPG beginnings in 1996, char products have been saved with the thought that they could serve as valuable soil additives. Pyrolysis char, a bi-product of GPG type of wood pyrolysis, has recently captured the attention of agronomists, environmentalists and economists in a rapidly growing International Bio-char Initiative (IBI) [22-24]. IBI looks upon cropland sequestering of CO2 as an important opportunity to mitigate climate change. Thus the need for large central and small distributed scale pyrolysis systems to convert waste from fast growing plant material to bio-char might soon be widely recognized. The IBI identifies how this agricultural-thermo-technology approach can provide a low cost method of pumping CO2 from the atmosphere and, sequestering it in long lasting black fertile cropland soils such as Terra Preta de Indio found in South America. It should be noted that whereas nature takes some 100 million years to make coal, a GPG converts biomass to bio-char in minutes. Pyrolysis converters of waste from high yield forestry and agriculture could provide a solar energy driven pump system to convert atmospheric CO2 into longed lived carbon amendments that can make very productive black soils. Indeed such an overall system of growing plants and thermally extracting its stored solar energy and a bio-char product, rather than be simply carbon neutral, could be the best possibility for achieving carbon negative. Bio-char has two main benefits: the extremely high affinity of nutrients to bio-char (adsorption), and the extremely high persistence of bio-char (stability) [22-24].
An advantage of the GPG is that its operator can control several operating parameters that influence char characteristics. For example, by controlling the auger rotation rate one can control the residence time of pyrolysis that usually gives something between slow and fast pyrolysis and influences the properties of the bio-char. Controlling the excess air used in the gas oven is one of several ways to control the temperature applied to the feedstock. Controlling the moisture content of the feedstock is an important way of influencing the char quality as well as the oven temperature. Blending various other organic or selected inorganic substances in the feedstock can strongly influence char quality. In the improved GPG, a small flight pitch is typically used at the bottom of the feedstock hopper and in the entrance half of the reactor. A larger flight pitch is used in the exit half of the reactor. The small flight pitch when full provides a useful block to the intrusion of air from the open feedstock hopper or the escape of pyro gas out of this entrance. The emptier long pitch flights near the exit of the reactor provide a hot chamber for the interaction of the char with the hot H2O in the pyro gases that arise as pyrolysis products or from feedstock moisture. The unfilled reactor exit flights facilitate the passage of the pyro-volatiles to the gas oven. The water-char reactions can be written asH2O+CnHmOp→CnHmOp+1+H2 which is a generalization of the well known steam gasification reaction H2O+C→CO+H2. The net effect is the oxygenation of the char and release of gases (CO plus H2) from the char particle producing thereby not only a better gas but also more pores in the char. The intimate contact of the hot pyro-gas with the hot volatizing feedstock on the exit side of the GPG reactor also fosters high temperature CO2 char reactions that can be written as CO2+CnHmOp→CnHmOp+1+CO. This is a generalization of the well known Boudouard reaction, CO2+C->2CO.
In effect, in the improved GPG an auger flight pitch arrangement is used to foster useful high temperature reactions with two big pyrolysis products (H2O and CO2) that usually are a problem in other pyrolysis arrangements. In the GPG they increase the pyro-gas yields and foster pore development in the emerging char. The char can serve as charcoal fuel, as a bio-char type soil amendment or, with more complete pore development, as activated carbon.
Studies of the pyrolysis of corn stover provide a good illustration of the usefulness of the ASEM in coping with the complexity of pyrolysis product yields for biomass type feedstock. A large body of experimental corn stover pyrolysis yields was measured with a Pyroprobe-FTIR system at Taiyuan University of Technology (TUT). They were made using a wide range of temperatures (T) and heating rates (r). A paper by Green and Feng [17] organized this data using a special case of the analytical semi-empirical model (ASEM)Y(T,r)=W/{1+exp(To−T)/D}2 where W=Wa+Wb In r, and To=Ta+Tb In r
Table 2 provides a small number of adjusted parameters that, with the formulas, give a reasonable account of the massive body of experimental data. This set of corn stover data was also organized with a traditional kinetic model (Arrhenius reaction rates) and comparisons were made between the two models. From the viewpoint of engineering applications of pyrolysis the ASEM proved simpler to use and more robust. In addition to the parameters for H2O, CO2 and CO, Table 1 gives the ASEM model parameters for 2 families of hydrocarbons and 5 families of carbohydrates.
TABLE 2Corn Stover parametersFamilyTaTbWaWbDCO247321.36.65−0.54155H2O50316.12.13−0.23135CO487206.5−0.56160paraffin54517.83.13−0.3115olefin571441.85−0.21145carbonyl42330.54.82−0.45115ether47129.65.16−0.5135aldehyde46125.44.65−0.31125alcohol50722.95.25−0.39165phenol51921.54.48−0.32165The paraffin family (CnH2n+2) that consists of CH4, C2H6, C3H8, C4H10, C5H12 etc. might also be considered to include H2 (n=0). The olefin family i.e. CnH2n, includes C2H4, C3H6, C4H8, C5H10, etc. Only seven families were measured in the Corn Stover study. Families identified in other ASEM studies include, acetylenes, dienes, formic acid, BTX aromatics, PNA, guaiacols, syringols. and sugars. References 11-15 and 20 give approximate formulas for the yields of individual family members. It cannot be over emphasized that pyrolytic reactors produce a very complex volatile brew whose detailed constituency is still beyond the predictive capability of today's science. Nevertheless, the phenomenological ASEM has served as an essential guide to the improvements of the GPG and its applications.
In this latter regard we might note that the improved GPG form lends itself to the production of activated carbon in a variety of ways. Firstly it makes use of pyrolytic water and carbon dioxide products for improved char formation. It is also simple to augment the water if needed by blending with damp feedstock. The operator can also augment the carbon dioxide by blending the feedstock with substances the release carbon dioxide at low temperature. Finally it is simple to blend the feedstock with inexpensive catalysts that foster the activation process.
While feedstocks suitable for pyrolysis include waste and agricultural materials such as wood chips, sawdust, granulated agricultural residues or energy crops, pine bark chips, pine needles, oak leaves, cogon grass, Christmas tree chips, football game waste, MRE waste, food court waste, old roof shingles, tire chips, solar dried sewage sludge, chicken or pig or cow manure, horse bedding, rat bedding, dried eutrophied lake muck and similar waste, blending of feedstock in the improved GPGs opens up a number of other GPG system applications. For example Green and Schaefer [9] have examined the conversion of lignite to useful soil additives by oxygenation with CO2. The improved GPG arrangement devoid of lock hoppers readily facilitate blending materials such as wood chips that give high CO2 pyrolysis yields with coal or lignite so that the lignite product comes out oxidized like Leonardite or Humalite, that are known to be valuable soil additives [9].